There are many common uses of transponders and readers as a means of identification. For instance, transponders are incorporated in car keys in conjunction with readers located in the vehicle to form a passive cryptologically safeguarded vehicle access system.
As a rule, the transponder contains a demodulator to permit demodulation of the signals received by the reader. The input of the demodulator is, as a rule, a signal processing circuit making the connection between the antenna of the transponder and the demodulator and converting the signal received at the antenna of the transponder into a form suitable for the demodulator.
One problem associated with transponders is that the input signal received by the reader exhibits very large fluctuations as regards its voltage level. For instance, amplitude shift keyed (ASK) signals may feature a peak-to-peak voltage level between 5 mV and 15 V. The reason for this heavy fluctuation in the voltage level is that the distance between the reader sending the signal and the transponder greatly differs. This is due to the fact that in the case of a passive vehicle access system the driver holding the key incorporating the transponder is at a distance away from the vehicle which greatly differs when he actuates the key.